Optical design of image stabilizing Cassegrain-like balloon-borne telescope

We describe the optical design of balloon-borne telescope with an image stabilizing secondary mirror which correct the pointing error of the attitude control system and enable a long-time exposure. We adopted a Cassegrain-like two-mirror system, and investigated the best mirror surface figure by surveying Conic constant (K) of the primary mirror from -1 to 0, where the secondary mirror is a high order aspheric surface to cancel spherical aberration at each K. In the small pointing error region, the classical Cassegrain system (K=-1) provides a wide field of view. However, in the large pointing error region, systems with large K provide a wide FOV. For λt (target wavelength) =50µm and 69cm aperture, a spherical primary system has acceptable optical performances when we assume the pointing error around 0.3 degree.